1. Technological Field of the Invention
The present invention relates to the field of manufacturing products, such as semiconductor chips, and more particularly, to a method and apparatus for selecting which products to inspect for defects during the manufacturing process.
2. Description of the Related Art
The manufacture of most products, such as wafers containing semiconductor devices, requires a number of discrete processing steps to create the product. For the example of wafers, a number of discrete steps are needed to produce a packaged semiconductor circuit device from raw semiconductor material. The starting substrate is usually a slice of single crystal silicon referred to as a wafer. Circuits of a particular type are fabricated together in batches of wafers called "lots" or "runs". The fabrication process creates regular arrays of a circuit on the wafers of a lot. During processing, the individual wafers in a lot may go through individual processing steps one at a time or as a batch. At the completion of wafer processing, the wafers are tested to determine circuit functionality. Later the wafers are sliced, the functioning products are packaged, and further testing occurs prior to use by the customer.
Data gathered during the course of wafer processing is used to diagnose yield problems and forms the basis of yield improvement efforts. For example, during semiconductor processing, particle contamination in the processing tool will normally have a deleterious affect on product yield. In order to collect data on particle contamination, a sensor may be used that senses the level of particle contamination in the processing tool chamber, in the outflow gas, or other area.
In known arrangements, this in-situ sensor for measuring a parameter or operating condition, such as particle contamination, is controlled independently of the processing tool. This sensor also has its own separate database for storing the sensor data that is collected during the manufacturing processing step performed by the processing tool.
Although in-situ particle monitors provide a 100% monitoring of particle deposition at the wafer level, they do not always accurately predict the level of contamination that is actually contained on the wafers. This is because different process recipes will have different counts of particles in the outflow of gas, without necessarily being too high for that particular process. Also, the particle count in certain regions of the processing tool may not accurately reflect the level of contamination on wafers currently being processed in the processing tool.
In order to obtain accurate information regarding the contamination of a wafer, the surface of an actual wafer is scanned (a "surface scan") using an inspection machine, such as the Inspex wafer scanning machine manufactured by Inspex of Boston. These wafers are normally selected at random, scanned for defects, and then the problem in the processing tool, if any, will then be addressed. This random selection of wafers to be inspected generates a number of problems. For example, wafers must be removed during the manufacturing process, which presents the possibility of adding more defects to the wafers. Also, this conventional method adds a step to the manufacturing process, increasing the cost, delaying the process, and adding more particles to the wafer.
Hence, although wafer scans are very reliable, to the extent that they may be used to shut down a processing line, the costs associated with the random selection and inspection of wafers effectively prohibits 100% sampling of the manufactured wafers.